1. Field of the Invention
The present invention concerns a method to produce x-ray-optical gratings, x-ray-optical gratings and an x-ray system for x-ray dark field imaging and for x-ray phase contrast imaging.
2. Description of the Prior Art
It is known to produce an x-ray optical grating as follows. An x-ray-sensitive layer with an electrically conductive cover layer is applied on a base plate and a grating structure is transferred by a lithographic method into the x-ray-sensitive layer, so exposed and unexposed regions are created. The exposed regions of the x-ray-sensitive layer is dissolved so that a grating structure remains; a metal is introduced into the grating interstices by electroplating. A negative imprint of a grating made of metal remains after removing the x-ray-sensitive material and the base plate. A grating made of a first material is produced with this negative imprint, wherein this grating having a number of periodically arranged grating webs and grating openings, and the grating openings are filled by electroplating with a second material.
Such a method for the production of x-ray-optical gratings to generate x-ray dark field exposures and x-ray phase contrast exposures is known from DE 10 2006 037 281 A1. The term “x-ray-optical grating” as used herein means a grating that has certain absorption properties with regard to x-ray radiation.
Significant technological requirements for such an x-ray-optical grating exist with regard to the precision of the height of the absorbing structures thereof, the aspect ratio, and mechanical stability. Generally such gratings are produced according to a technique known as the LIGA method, from the German acronym for Röntgen-Lithographie, Galvanik, Abformung (x-ray lithography in English). In this procedure, a grating structure is first created in an x-ray-sensitive material via partial exposure with, for example, parallel synchrotron radiation, in which grating structure a metal is introduced by galvanic deposition. After the removal of the x-ray-sensitive material, this metal represents a negative with which a grating is produced from a material with low absorption coefficient. To improve the absorption properties of this grating, the grating openings are filled, by electroplating, with a different material that has a higher absorption coefficient.
In the x-ray-optical gratings produced according to the LIGA method, in particular in absorption gratings with a high aspect ratio, a number of parameters (for example the surface roughness, and what is known as the bath temperature) affect the deposition process of the second metal in the electroplating and lead to different growth heights within the grating gaps. The height of the filling between the individual grating webs can vary in some cases by up to 10-20%. These variances lead to a degradation of the measurement signal in x-ray dark field imaging and in x-ray phase contrast imaging, since the bands of generated high and low emission then deliver minima or maxima of different magnitudes that degrades the acquisition quality.